This invention relates generally to microfluidic devices having microfluidic channels with integrated ultrasonic transducers for measuring the temperature of fluids in said channels and more particularly to such devices employing ultrasonic transducers such as integrated micromachined ultrasonic transducers (MUTs) or piezoelectric transducers.
The large investments in the microelectronics industry converted integrated circuits laboratories into machine shops where miniature electromechanical systems are designed and built. Electromechanical as well as electro-optical systems have been miniaturized and used in many different applications. In the same fashion, miniaturization is presently applied in the field of microfluidics. Microfluidics technology provides the advantage of being able to perform chemical and biochemical reactions and/or separations with high throughput low volumes. Microfluidic systems employ microchannels formed in substrates or chips in which chemical and biochemical materials are transported, mixed, separated and tested.
One of the parameters measured such microfluidic systems is the temperature of the fluids and the change in temperature of fluids as a result of any reactions carried out in the channel.
A typical method for temperature measurement involves using a thermocouple embedded in the substrate, preferably close to the fluidic channel. However, this method measures the average temperature of the substrate and fluid. In most cases, the channel is so small that the thermocouple reading is dictated primarily by the substrate temperature.
There is a need for directly measuring the temperature of fluids in microchannels.
It is an object of the present invention to provide a system and method for measuring the temperature of fluids in a channel using ultrasonic transducers.
It is another object of the present invention to provide a system for measuring the temperature of fluids in a channel using integrated ultrasonic transducers such as micromachined capacitive or piezoelectric transducers.
Micromachining permits fabrication of ultrasonic transducers having sizes compatible with the microchannels of microfluidic devices. Furthermore, the transducers can be integrated into the microchannels. Capacitive micromachined ultrasonic transducers (cMUTs) operating both in air and water are known and described in U.S. Pat. Nos. 5,619,476; 5,870,351 and 5,894,452. In both air and water, a Mason electrical equivalent circuit is used to represent the transducers and predict their behavior (W. P. Mason, Electromechanical Transducers and Wave Filters (Van Nostrand, N.Y., 1942)). These transducers are fabricated using standard IC processes and have been integrated with signal processing electronics to form an integrated system. In the article entitled xe2x80x9cHighly Integrated 2-D Capacitive Micromachined Ultrasonic Transducersxe2x80x9d appearing in IEEE Ultrasonic Symposium Proceedings pp. 1163-1666, 1999, S. Calmes et al., describe the fabrication of cMUTs with through wafer connections so that they can be flip-chip bonded to chips having signal processing electronics. Alternatively, the processing electronics can be implemented on the same silicon wafer avoiding the through wafer via structure. The dynamic range and bandwidth of cMUTs surpass their piezoelectric counterparts while being completely compatible with microfluidic chip fabrication processes.
Capacitive micromachined ultrasonic transducers (cMUTs) with dimensions of 100 xcexcm or less are fabricated on the walls of the fluidic channels and operate in the 1-100 MHz frequency range. The cMUTs are surface micromachined to have a low surface profile, permitting undisturbed fluid flow. These transducers enable in-situ measurements of temperature of the fluid in the channel by measuring the velocity and/or attenuation of sound waves in the fluid in the channel.
Ultrasonic transducers having the above-described characteristics can also be micromachined from piezoelectric material. They may be formed by thin film deposition such as sputtering, sol-gel deposition or other types of physical or chemical deposition. Through wafer interconnects can be used with piezoelectric transducers.
The present invention provides a temperature measuring system in which ultrasonic transducers fabricated in one wall of the channel or placed on the substrate opposite the channel generate ultrasonic waves which reflect from one or both walls of the channel and provide an output signal representative of the velocity of the sound waves or attenuation or both in the fluid. The signals are then processed to provide the fluid temperature.